1. Field of the Invention
The present invention relates generally to a semiconductor chip assembly and, in particular, to an apparatus that uses a flexible, silicone elastomer adhesive to directly join a chip or plurality of chips to one or several cooling members.
2. Description of Related Art
Semiconductor devices are continually becoming smaller and more dense with the evolution of new technology. However, increases in circuit density produce corresponding changes in overall chip packaging strategies in order to remain competitive. Chip and chip carrier manufacturers are therefore constantly challenged to improve the quality of their products by identifying and eliminating problems, reducing package size and weight, decreasing package costs and providing improved thermal efficiencies with new generations of advanced devices. Whereas significant improvements are being made to eliminate systematic problems by reducing process variability, process improvements alone are not sufficient to eliminate all the problems which affect both performance and reliability.
FIG. 1 illustrates a currently employed method and apparatus for sealing a metal cap 11xe2x80x2, to a ceramic substrate 12, by means of a sealant or adhesive bond 13. Typically, chip 14, is first secured to a substrate 12, via a plurality of solder balls 15 on pads 16 that reside on the top surface of the substrate 12. Substrate 12 could also have one or more electronic devices 17 such as a decoupling capacitor 17 that is also electrically connected to substrate 12, via metallized pads 16 and either solder balls or surface mountable solder. For some applications solder balls 15 and pads 16 could be encapsulated with a polymeric underfill material 18. A semi-liquid or paste type thermally conductive material 19 is usually applied over the exposed surface of chip 14 such that a direct thermal contact is made between the chip 14 and the cap 11xe2x80x2 when cap 11xe2x80x2 is placed so as to cover chip 14. A cap adhesive sealant 13 is typically provided in order to secure cap 11xe2x80x2 to the substrate or module 12. Heatsink 20 can be secured to cap 11xe2x80x2 using a heatsink adhesive 21. Substrate 12 is typically secured electrically and mechanically to a mother board, card or socket 22, via I/O (Input/Output) means 23, such as pads, pins, solder balls, solder columns, etc.
Cap 11xe2x80x2, typically fashioned from metals or ceramics having either high thermal conductivity and/or matched thermal expansivity to the chip carrier, is placed over chip 14 and is permanently secured to the surface of substrate 12. As shown, cap 11xe2x80x2 has sidewall portions 11a and usually completely seals the sides of the chip module from the surrounding environment. This is done primarily to prevent mechanical and chemical injury to chip 14, solder balls 15, decoupling capacitors 17, underfill 18 and any exposed metallurgy or circuitry on the substrate 12. It is well known that a leak in cap 11xe2x80x2 may result in irrecoverable module yield losses and degrade expected reliability performance under actual use conditions. A picture-frame type area on the top surface of the substrate 12 is required to specifically seal cap 11xe2x80x2 to substrate 12 using cap sealant 13. The width of this frame type will vary as a function of overall substrate size, using experimentally derived data, but includes the actual seal area in addition to associated required clearances 16, 17 around the seal to eliminate sealant runover or mechanical damage to devices during assembly. Therefore, the placement of all devices, such as, for example, chips 14, decoupling capacitors 17, is restricted to be within this picture frame area, which is typically 50 percent, of the area that would otherwise be available for additional or larger devices. Additionally, cap 11xe2x80x2 typically adds between 30 percent and about 50 percent to the overall height of the module. Furthermore, the presence of cap 11xe2x80x2 adds additional weight to the completed or assembled module.
The entirely enclosed internal cavity 28 created by the cap and seal will tend to trap moisture. This leads to high internal pressures and related damage during high temperature (above 100xc2x0 C.) operations, such as joining the module to a mother board. This phenomenon, known in the industry as xe2x80x9cmoisture sensitivity,xe2x80x9d drives added cost through special handling and preparation procedures, such as dry-bagging with dessicants and ambient exposure time limits to minimize the amount of moisture in the chip carrier prior to high temperature processes.
Thermal compound 19 must be placed between chip 14 and cap 11xe2x80x2 to provide an efficient heat transfer path via the heatsink adhesive 21 to the heatsink 20. The thermal compound is typically of semi-liquid or paste consistency to absorb the large thermally induced strains associated with this xe2x80x9cdoubly-connectedxe2x80x9d structure before they are transmitted to and damage the chip 14.
In some cases, thermally conductive epoxies have been used to provide a better thermal contact between the chip and the heat sink, while others have used thermally conductive pastes, greases and/or oil films. For example, U.S. Pat. No. 5,367,193 discloses use of Dow Corning 340 heat sink compound, which is a grease-like silicone material filled with metal oxide filler, in conjunction with a multistructural mechanical support. All of these methods suffer from one or more drawbacks, such as poor thermal performance, brittleness at low temperatures, material flow or movement over time, or the need for secondary mechanical support.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus which has a thermally conductive mechanically robust path between at least one chip and at least one cooling member.
Yet another purpose of the invention is to increase the available area on the substrate or the chip carrier for device joining, for example, active devices, such as chips, or passive devices, such as capacitors, resistors, etc.
Another purpose of the invention is to provide a method and apparatus to support high cooling member weights, for example, lid plus heat sink assemblies of at least 100 grams.
Another purpose of the invention is to provide a method and apparatus for ensuring the mechanical and operational integrity of the bond between devices and the cooling member under typical use conditions such as gravity, mechanical shock, vibration, high temperature with humidity and repeated thermal expansion/contraction cycles due to temperature variation.
Still another purpose of the invention is to provide a method and apparatus that will minimize thermal performance degradation over the chip carrier life.
Still yet another purpose of the invention is to provide a method and apparatus that will absorb thermally inducted strain without damage to the chip carrier or associated devices.
Yet another purpose of the invention is the ability to rework or repair the completed or assembled module in a simple, efficient manner. still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects, which will be apparent to those skilled in the art, are achieved in the present invention which provides in one aspect a chip assembly comprising a substrate having an upper surface for carrying one or more semiconductor chips, at least one semiconductor chip mounted on the upper surface of the substrate and a cap covering the semiconductor chip and the upper surface of the substrate. A silicone adhesive is placed between the semiconductor chip and the cap. The adhesive has sufficient bond strength to secure the cap to the chip without additional mechanical constraint while providing a direct thermally conductive path and permitting sufficient heat flow from the chip to the cap to maintain steady state operation of the semiconductor chip.
The invention is useful where the cap has a coefficient of thermal expansion significantly greater than that of the semiconductor chip. The cap may extend horizontally outward of the chip, and the chip or the silicone adhesive may be at least partially exposed to the environment outside of the assembly between the cap and the substrate. The chip may be secured to the substrate by an electrical connection selected from the group consisting of solder balls, solder columns, low melting point solder and high melting point solder, and the electrical connection may be encapsulated with an underfill material. In such case, the cap may extend horizontally outward of the chip, and the underfill material is at least partially exposed to the environment outside of the assembly between the cap and the substrate.
The cap preferably has a substantially flat upper free surface and may comprise a heat sink for the chip. There may be a plurality of semiconductor chips mounted on the upper surface of the substrate having different heights, such that the cap covers the semiconductor chips and the upper surface of the substrate, and the silicone adhesive is disposed between the semiconductor chips and the cap at different thicknesses dependent on the height of a chip. The assembly may also include at least one additional electronic device secured to the substrate and disposed under the cap.
In one embodiment, the cap includes a support member extending horizontally beyond a peripheral edge of the chip and downward to the substrate and secured thereto.
The assembly may further include a plurality of semiconductor chips mounted on the upper surface of the substrate, wherein the cap covers the semiconductor chips and the upper surface of the substrate such that the silicone adhesive is disposed between fewer than all of the semiconductor chips and the cap.
In another aspect, the present invention provides a method of providing a direct thermally conductive path between at least one chip and at least one heatsink cap. The method comprises initially providing a substrate having an upper surface for carrying one or more semiconductor chips and at least one semiconductor chip mounted on the upper surface of the substrate, and a heatsink cap for covering the semiconductor chip and the upper surface of the substrate. The method then includes the steps of applying a silicone adhesive between the semiconductor chip and the cap, securing the cap to the substrate by means of a mechanical fixture, curing the silicone adhesive, and removing the mechanical fixture. The cured adhesive has sufficient bond strength to secure the cap to the chip without additional mechanical constraint while providing a direct thermally conductive path and permitting sufficient heat flow from the chip to the cap to maintain steady state operation of the semiconductor chip.
The method may further include the step of securing at least one portion of the chip to a substrate using at least one electrical connection.
The silicone adhesive preferably. comprises a primeness, two-part polysiloxane-based adhesive made by reacting polydimethyl siloxane, an organosilicon compound, a polysiloxane, and a silane, in the presence of a catalyst, and optionally includes a thermally conductive material. The silicone adhesive is preferably cured to support a cooling member mass of at least 100 grams.